Apparatus for forming and cooling thermoplastic resin pellets



Jan. 27, 1,970 C TAKuRo w-AI-ANABE :ET A-L 354913829 APPARATUS Foa FORMING AND COOLING THERMOPLASTIC RESIN PELLETS Original Filed June 11, 1965 United States Patent O 3,491,829 APPARATUS FOR FORMING AND COOLING THERMOPLASTIC RESIN PELLETS Takuro Watanabe and Kazuhiro Toyama, both of Goi Kaigan, Chiba Prefecture, Ichihara-shi, Japan Original application June 11, 1965, Ser. No. 463,231, now Patent No. 3,415,917, dated Dec. 10, 1968. Divided and this application June 25, 1968, Ser. No. 739,684

Claims priority, application Japan, June 18, 1964,

U.S. Cl. 165--47 3 Claims ABSTRACT OFv THE DISCLOSURE A process and apparatus for extending and cutting a heated thermoplastic resin to form highly viscous pellets which are then introduced into a whirlstream 'of cooling water to cool the pellets to a non-sticky state.

This invention relates to an apparatus for manufacturing pellets of thermoplastic resin and is a divisional of copending application Ser. No. 463,231, filed June 11, 1965, now U.S. Patent No. 3,415,917 and granted ec. 10, 1968. More particularly, it relates to an apparatus for manufacturing pellets of thermoplastic resin by cooling hot, highly viscous, heat-shaped pellets of a thermoplastic resin for a short time, using a comparatively small amount of a cooling medium, preventing the pellets from sticking to each other or being fused into each other or sticking to the wall of the apparatus; and thus manufacturing them in a uniform shape in a very effective manner.

Thermoplastic resin is produced in various forms, de-

pending upon the method of manufacturing that is employed. For example, it is obtained in the form of an indefinite solid or mass by block polymerization; in globular shapes of various sizes by pearl polymerization; in particles or powder by suspension or emulsion polymerization; or as a solution in an organic solvent by solu tion polymerization.

The resin in such shapes as mentioned above is inconvenient to handle, and it is in a form which is especially difficult to charge to a mo-lding machine, steadily or quantitatively. Thus, it is generally shaped into a xed form, for example, in pelleted form for such purpose.

Pellets have been produced by crushing a resin mass into grains of a uniform size. Also, resinous powder has been shaped into the form of a plate, or stamped out into a fixed shape, and then crushed or cut after it has been cooled. However, these methods cannot be operated on a large scale. As an ecient method on a large scale, it is most common to extrude the resin with an extruder into string-shaped material `which is then cut into a suitable length. This method is very popular, making it pos sible to obtain easily the pellets of uniform shape and size. In making pellets with the use of an extruder, there are two known methods: (1) the cold cut method, in which the extruded resin is cut after being cooled and hardened; and (2) the hot cut method in which the resin is cut at the surface yof the die or extruder immediately after extrusion without being cooled. The cold cut method is troublesome to operate and requires force for cutting. The hot cut method is employed widely as an eflicient method on a large scale since the cutting is easy and the apparatus is simple.

As a special pelletizing method, when a water-insoluble resin is obtained as a solution in an organic solvent and especially when the organic solvent does not mix with water, the solution of the resin is heated while being mechanically agitated and dispersed in water. The solvent ICC is eliminated by evaporation and the resin particles are precipitated in the form of a suspension in water. This is also one of the methods that may be employed for pelletizing thermoplastic resin.

On pelletizing thermoplastic resin by the hot cut method, polyvinyl chloride containing no plasticizer, for example, is hardly viscous at its extruding temperature, so there is no need for cooling it. Thus, after it is cut in air, it may be gathered in a container. This is, however, a rare case. Generally such thermoplastic resins as the polymers of vinyl acetate, methyl acrylate, ethylene and propylene and their copolymers must be cooled to a suitable temperature where their viscosity is reduced or the pellets will be fused to each other or to the wall of the a-pparatus or container, resulting in a troublesome or inetlicient operation.

Various methods have been proposed for cooling pellets in the hot cut method. One method is to throw pellets in a vessel filled or overflowing with a cooling medium. In this case, however, if the resin pellets have a lower specific gravity than the cooling medium, they float on its surface, forming condensed masses which fuse into each other and which will usually fuse to the wall of the container. In order to prevent such fusion, the cooling medium must have a wide free surface. If the resin is of greater specific gravity than the cooling medium, the precipitated pellets, that are not cooled sutliciently, become deposited and fused to each other at the bottom of the cooling vessel.

Another method is very similar to the abovementioned method, except that an agitator is installed in the cooling vessel. In this case, the effect of cooling the pellets is increased and their cohesion is readily prevented. However, they beco-me fused to the blades of the agitator and to the wall of the cooling vessel. A further method is to extrude a thermosplastic resin directly into a cooling medium, in which it is cut (Japanese patent publication No. S32-439). In this method, the end `of the extruder which is heated to a higher temperature must be insulated thermally from the vessel containing a cooling medium of a lo-wer temperature. The temperature of the resin in the die of the extruder is apt to drop, resulting in higher resin viscosity which, in turn, brings about a large power consumption and complication Iof the apparatus and its operation. In some cases, the pellets may become fused to each other in the cooling medium for the same reason as that of the first method.

.A still further method is one in which fused particles of resin that are cut in air are made to fall and strike a slope on which poured water is running and the flow of waterchanges their direction by and discharges them out of the cutter (Japanese patent publication No. 837-15239). This method employs a simple apparatus and can achieve the desired operation to some extent on a small scale. However, the pellets are not cooled sufficiently and the pellets separated from the flowing fluid are apt to become fused to each other. According to experiments carried out by the present inventors concerning the last method, when crystalline polypropylene is extruded at about 200 C. and cut immediately into pellets, the pellets cannot be cooled sufciently even with the addition of a large quantity of water. When separated from the cooling water by means of a filter, the pellets become fused by more than 10 percent. For preventing the above, it has been found that a very long water course is required for cooling the pellets.

An object of the present invention is to provide an apparatus for pelletizing thermoplastic resin eiciently by the hot cut method.

Another 'object is to provide an apparatus for manufacturing pellets in a uniform shape.

A further object is to provide an apparatus for cooling pellets eiciently with a comparatively small amount of a cooling medium.

The present invention is an apparatus for pelletizing thermoplastic resins which apparatus comprises means for throwing highly viscous, high temperature hot cut pellets of a thermoplastic resin that are cut in air directly into the stream of a cooling medium and then whirling the stream, thereby causing the pellets to whirl together with the medium until they are cooled and become substantially non-viscous before they are separated from the cooling medium.

Generally, the thermoplastic resin is a very poor heat conductor. When pellets of the thermoplastic resin are cooled by placing them in a cooling medium for a short time, only their surfaces are cooled and lose stickiness, but the interior of the pellets is still at a high temperature. If the pellets in such a state are separated from cooling medium, heat retained in the inside is conducted to their surfaces and as a result, they become viscous again. If the pellets are solidified only at the surface, but are still viscous in their interior, and contact each other or the surfaces of other Solid substances, for example, the wall of a vessel, the pellets become fused to each other when they are stationary, or when they collide against each other or against the wall of a vessel, even if they exist in a liquid at a low temperature at which they are not viscous any longer.

The following is a description of the apparatus of the present invention in reference to the attached drawings.

FIGURE 1 is a side elevational view illustrating one type of apparatus according to the present invention. FIGURE 2 is a perspective view of the apparatus shown in FIGURE 1. In FIGURE 1, the main body of the cooling apparatus 5 has an opening 11 for receiving the pellets at its top and a nearly cylindrical device 6 for whirling the cooling medium. The bottom of unit 5 may be designed so as to be at the same level or sloped toward the whirling device 6. The bottom of unit 5 need not be necessarily flat but may be curved. An adequate number of the ejecting pipes 7a and 7b for the cooling medium are installed opposite whirling device 6. A plurality of jets 8a and 8b each having a suitable diameter are fixed in ejecting pipes 7a and 7b or slits of a suitable breadth are installed in the longitudinal direction of tube 7a and 7b, through which the cooling medium is ejected toward the lower edge of the opening 9 of the whirling device 6. In this case, the cooling medium is ejected in a straight line from the jets to the lower end of the opening 9 as shown by an arrow mark c in FIGURE l. However, the cooling medium may also pass along the bottom of the main body 5 as shown by the arrow mark d in FIGURE l. Further, it may also pass in both directions simultaneously.

The hot pellets of thermoplastic resin which have been extruded from an extruder 15, cut with a cutter 14 and while still in a semi-molten or plastic state are thrown in these currents and swept away thereby toward device 6. The stream comprising the cooling medium accompanied by the pellets enters device 6 through the opening 9, and flows along the wall in a whirling movement in the direction of the arrow mark e. The stream moves whirling together with the pellets to the discharge end 10, and is discharged out of the apparatus. The cooling medium is separated from the cooled pellets in a separator (not shown). After being separated from the pellets, the cooling medium is disposed of or recirculated for use, as is, or after a part of it is exchanged with a new cooling medium. A discharge may be installed at each of the two ends of the whirling device v6, Le., also on surface 5, or only at one side as shown in FIGURE 2. In either case, the semicylindrical portion of the whirling device 6 may be either level or may be sloped slightly so as to go downward at a suitable angle towards a discharge end.

Further, it is advantageous to install the whirling device in such a way that the discharging stream of the cooling medium in this device makes a suitable obtuse angle with the direction of the whirling current in the main body 5. By such arrangement it is possible to cause the pellets to flow smoothly and thus prevent them from being retained in this device.

The diameter and length of the whirling device 6 can be determined easily from the quantity of cooling medium and the residence time required depending upon the properties of the thermoplastic resin being manufactured. It is also easy to determine the velocity of ow of the cooling medium on the basis of the foregoing factors.

The cooling medium supplied through the pipe l6 must be given the necessary energy in advance for it to whirl in the whirling device 6. It is determined from the velocity of ow of the cooling medium which is ejected through the pipe 7a or 7b. The inventors have conrmed experimentally that when water of normal tap temperature is used, the suicient velocity of water flow is above about 3 meters per second at the opening 9 if the diameter of the whirling device is 300 millimeters, and above 4.5 meters per second if it is 600 millimeters. The flow of the cooling medium being ejected through the pipe 7a or 7b should cover the whole bottom of the main body 5 (not leaving any part of it dry) and need not necessarily be uniform all over the bottom. However, it is desirable that a large quantity of the cooling medium be directed at the main body of the pellets by adjusting the pitch of the ejecting holes or slits. Whatever kind of a cooling device may be employed, it is desirable to flow a suitable quantity of the cooling medium on a part of all of the wall to keep it wet because if the pellets contact the upper part of the wall just after being cut while in a semi-molten state, they will become adhered to the walls as well as each other.

The time of contract between the cooling medium and the pellets may vary more or less according to the kind of the resin, the cutting temperature, the size of the pellets and the kind, temperature and quantity of the cooling medium. However, when polypropylene is cooled with water below 50 C., a time of contact of about 1.5 seconds is sufficient.

Needless to speak of, the apparatus of the present invention can fbe applied not only to thermoplastic resin itself but also to the manufacture of pellets of any composition which is obtained by mixing a suitable stabilizer, dye, pigment, ller, plasticizer or diiferent thermoplastic resin with it or by their combination.

EXAMPLE 1 The apparatus illustrated in FIGURE 1 of which the bottom was about 1 meter long and 1.2 meters wide and a whirling device was 30 centimeters in diameter, was employed. Water of about 40 C. was ejected at a rate of 50 cubic meters per hour through many small holes, each about 3 millimeters in diameter, from a pipe 7a or 7b to the lower edge of an opening 9 of the whirling device 6. In this case, the speed of water flow at the entrance of the whirling device was about 6.5 meters per second.

A powder of crystalline polypropylene, about 140,000 in molecular weight, blended with a small quantity of a phenolic typey stabilizer was charged at a rate of 800- 1,'000 killigrams per hour to a hot cutting machine with an extruder having a maximum temperature adjusted to about 220 C. The extruded polypropylene is cut in pellets, each about 3 millimeters in diameter and about 1.5 millimeters long and is thrown immediately into the owing Water of said cooling apparatus. The mean residence time of the pellets in the cooling apparatus was about 2 seconds. Even if the pellets were discharged from the cooling apparatus and separated immediately from water, they did not adhere to each other. After a continuous operation of about 60 hours, it was observed that polypropylene did not adhere at all to the wall of the apparatus.

Hot-cut polypropylene pellets similar to the above, just after being cut, were thrown at a rate of about 800 kilograms per hour into a vessel, about 1.2 meters wide and about 1.5 meters long, containing water of about 50 centimeters in depth to which water at about 30 C. was supplied continuously at a rate of 50 cubic meters per hour. Ten percent of the pellets adhered to each other and three percent adhered to the wall of the vessel.

EXAMPLE 2 Polystyrene pellets were charged to the same cooling apparatus as that of Example 1, in which water of about 29 C. was ejected at a rate of about 40 cubic meters per hour. The pellets, which were each about 3.2 millimeters in diameter and about 2 millimeters thick, about 85,000 in molecular weight, lmade with a hot cutting machine, were charged at a rate of about 800 kilograms per hour. The pellets which were separated after contacting with the cooling water for 2 seconds were not adhered to each other. After a continuous operation for about hours, it was Vobserved that polystyrene did not adhere to the wall of the cooling apparatus.

EXAMPLE 3 A powder of poly(methylmethacrylate) compounded with 0.1 percent by weight of copper phthalocyanin blue was supplied at a rate of about 650 kilograms per hour to the apparatus of Example 1 and made into hot-cut pellets at 215 i5 C. The pellets were cooled using Water of 35 C. at a rate of about 50 cubic meters per hour. After cooled for about 2 seconds, the pellets did not adhere to each other.

What is claimed is:

1. An apparatus for cooling pellets of thermoplastic resin which are hot and viscous immediately after extrusion and cutting, comprising a case having a nearly fiat bottom, a first sidewall defining an opening for introducing the pellets near the top of the case, a cooling medium pipe near the first sidewall and having a plurality of cooling water ejecting holes to eject a cooling stream toward a cylinder thereby carrying pellets falling from the pellet inlet opening to the cylinder, said cylinder being attached to the lower end of a second sidewall opposite said rst sidewall with the axis of the cylinder at a nearly horizontal position, less than half of the surface area of the cylinder Ebeing open to the inside of the case to define an elongated slit for receiving incoming pellets as well as the cooling medium, whereby the ejected stream of cooling water flows together with the pellet toward the slit and after passing through the slit said Water ows along the inside wall of the cylinder to form a whirl stream in the cylinder by mixing with a part of the stream incoming from the opposite direction, and at least one opening deiined by at least one of the bottoms of the cylinder to discharge the cooled pellets and the cooling medium.

2. An apparatus for cooling pellets of thermoplastic resin comprising a case having a nearly flat bottom, a first sidewall delining a pellet inlet opening near the upper end of the case, a second sidewall opposite the first sidewall, a cylinder attached to the lower end of the second sidewall, the axis of said cylinder being in a nearly horizontal position with less than half of the interior surface area of the cylinder open to the inside of the case, and a cooling medium conduit disposed near said first sidewall, said conduit having a plurality of cooling medium ejecting holes directed toward the cylinder, said cylinder having at least one outlet opening at one end thereof for discharging the cooled pellets and the cooling medium.

3. An apparatus for cooling pellets of thermoplastic resin comprising a case having v a nearly flat bottom terminating in end edge portions a first sidewall extending upwardly from one side edge of said bottom, said first sidewall being provided with an opening for introducing pellets into said case a second sidewall opposite said iirst sidewall and positioned so that one end edge thereof is spaced from and adjacent the other end edge of said bottom t0 provide a horizontally disposed outlet in said case a conduit for supplying a iiuid cooling medium to said case mounted adjacent said first sidewall, said conduit having openings directed toward the spaced end edges of said bottom and said second sidewall a horizontally disposed cylindrical member having an opening extending lengthwise in the sidewall of said cylinder and an outlet opening at one end thereof, said cylinder sidewall opening being less than one half the circumference of said cylinder member and having the edges of said cylinder sidewall opening secured to respective sidewall edges of said bottom and said second sidewall to provide an opening for the flow of cooling medium and pelletstherein into said cylinder in a whirling manner.

References Cited UNITED STATES PATENTS 3/1966 Kogan 165-107 2/1968 Cross et al.` 165-107 U.S. C1. X.R. -119 

